This invention generally relates to the measurement of particulate matter suspended in a gaseous fluid medium, such as particulate matter suspended in ambient air. More particularly, this invention concerns an apparatus and method for controlling the thermodynamic conditions of a sample gas stream in order to more accurately measure suspended particulate matter in the gas stream, preferably under near real-time conditions.
The measurement of particulate matter in ambient air is important for a variety of reasons, the most important of which is related to health effects. Suspended particulate matter is known to produce a variety of deleterious health effects when inhaled. As a result, regulatory agencies around the world require monitoring of the levels of particulate matter. The levels are measured in terms of concentration, i.e., micrograms of particulate matter per cubic meter of air. Reference techniques for this measurement are presently defined in terms of a mass measurement utilizing a filter medium to capture the particulate matter and the total volume of air which has been filtered by the medium over a given period of time. There are various means available to unambiguously determine the flow rate through the filter over time (and hence the volume of the air sampled), but surprising, the mass measurement is not straightforward due to the complex nature of ambient particulate matter which results in unstable mass deposition on the filter.
Both direct and indirect measurement techniques have been employed in an effort to quantify particulate matter mass. Each method which has been developed to date, however, has limitations in obtaining a measurement of the actual mass of particulate matter as it exists in its suspended form in ambient air. Direct mass measurements as represented by weighing material captured on a substrate such as a filter media are susceptible to volatile losses which are not easily quantifiable. Indirect methods such as light scattering measurements, on the other hand, are inherently inaccurate as there is no physical connection between other properties of particles and particle mass.
As a result of these difficulties, the current reference method in the United States is a manual method dependent technique which does not necessarily provide a completely accurate measure of particulate matter as it actually exists in its undisturbed state in the air. The manual reference method consists of: (1) filter equilibration under a pre-defined range of temperature and humidity conditions (i.e., currently 20xc2x0 C. to 23xc2x0 C. xc2x12xc2x0 C. and 30% to 40% xc2x15% relative humidity for PM-2.5 standard and 15xc2x0 C. to 30xc2x0 C. xc2x13xc2x0 C. and 20% to 45% xc2x15% for PM-10 standard); (2) a pre-collection weighing of the filter; (3) the installation of the filter in a manual sampler and the sampling of ambient air (for a 24-hour period); (4) the removal of the filter from the sampler and a post-collection conditioning under the same equilibration conditions for the filter as performed pre-sampling; and finally, (5) post-collection weighing of the filter to determine the mass captured on the filter media.
While sampling, the manual reference method does not consider the sample gas stream or ambient air temperature or humidity conditions. In effect, this method uses only the pre-defined sampling temperature and humidity conditions before and after sampling. This method does not compensate for condition changes during sampling. While this methodology is intended to provide a consistent basis for the generation of standardized 24-hour sampling results, unfortunately it does not. First, it may give an inaccurate measurement due to failure to consider sample gas stream conditions. And second, it presents difficulties when other measurement techniques utilized in automated, near real-time samplers, for example, are compared to it.
Automated samplers have significant advantages over manual reference method samplers in that they can provide near real-time measurements at great labor cost savings (i.e., the post-collection measuring and calculating steps in the manual method are eliminated because measuring and calculating are conducted in real-time by the automated sampler). Examples of such automated samplers include inertial mass measurement ambient particle monitors and beta attenuation monitors. Further, due to their near real-time measurements of mass, these samplers respond to volatile components in ways which are not directly comparable to the reference manual method.
However, like the manual reference apparatus and method, existing automated samplers may not fully consider the temperature and humidity conditions of the sample gas during sampling while making their real-time measurements. Also, the integrated 24-hour results from the automated samplers may not always agree closely with the 24-hour single measurement from the manual reference method. Thus, although the automated samplers provide a more real-time measure of particulate matter present in a sample gas stream, they still have drawbacks, including their potential inability to absolutely conform to the manual reference method all the time.
Accordingly, a method and apparatus is needed to mitigate any differences between the results of the manual reference method and the automated sampler method. Further, a sampling apparatus and method is needed that can enable near real-time sampling and/or automated sampling under near reference manual method type equilibration conditions in order to obtain near real-time and/or automated results, which will enable even more consistently comparable particulate matter measurements than are available through the manual reference and existing automated apparatus and methods.
The existing apparatus and methods for measuring particulate matter suspended in a gaseous medium do not offer the flexibility and inventive features of the present sample gas stream conditioners, system and methods. As will be described in greater detail hereinafter, the features and advantages of the present invention differ from those previously proposed.
According to the present invention, there is provided a sample gas stream conditioner assembly for use with a particulate matter measuring instrument and a system and methods incorporating the same. The sample gas stream conditioner assembly comprises a sample gas stream conduit for receiving a sample gas stream and conducting the sample gas stream to the measuring instrument. A moisture exchanger and a humidity sensor are in communication with the conduit and a humidity control device is in gas flow relationship with the moisture exchanger. The moisture exchanger, humidity sensor and humidity control device serve to condition the sample gas stream, wherein the humidity level of the sample gas stream is regulated to obtain a selectable humidity level as the sample gas stream is conducted to a particulate matter collector of the measuring instrument.
Another feature of the invention relates to the preferred elements of a heat exchanger and a temperature sensor in communication with the conduit and a temperature control device in communication with the heat exchanger. These three elements serve to further condition the sample gas stream, wherein the temperature of the sample gas stream is regulated to obtain a selectable temperature as the sample gas stream is conducted to a particulate matter collector of the measuring instrument.
Still another feature of the invention concerns an ambient condition sensor system which can be integrated with the conditioner assembly to further assist in conditioning the sample gas stream.
Yet another feature of the invention is an automatic control unit wherein various components of the invention are automatically operated and in communication with one another to provide near realtime particulate matter measurements in the field, preferably under selectable constant humidity and temperature conditions.
This invention also contemplates a conditioning method in which a humidity level at a particulate matter collector in a particulate matter measurement instrument is regulated and various approaches for accomplishing this objective.
In accordance with the following, it is an advantage of the present invention to be able to collect particulate matter and maintain a sample gas stream at selectable humidity and/or temperature conditions, thereby bringing results between automated and manual reference methods and apparatus into closer agreement.
A further advantage is to maintain constant selectable humidity and temperature conditions to enhance apparatus and water mass stability thereby reducing both real and apparent mass changes in the particulate matter due to humidity changes or temperature effects.
Yet a further advantage is to be able to collect and measure particulate matter at lower temperatures without the undesired effects of humidity, thus also, reducing the loss of non-water volatiles that contribute to a more consistent particulate matter mass measurement.